Mention more ffmpeg versions in the README

Author: Polochon-street

README.md

@@ -4,7 +4,8 @@
 
 This is a fork of the abandoned [ffmpeg](https://crates.io/crates/ffmpeg) crate by [meh.](https://github.com/meh/rust-ffmpeg).
 
-Currently supported FFmpeg versions: 3.4.x through 4.4.x.
+This crate is currently in maintenance mode, and aims to be compatible with all of FFmpeg's versions from 3.4
+(currently from 3.4 til 7.0)
 
 Build instructions can be found on the [wiki](https://github.com/zmwangx/rust-ffmpeg/wiki/Notes-on-building).